In a communication system, a signal is transmitted through an antenna assembly which is often remotely located in the fixed part of a communication system. A similar antenna assembly is utilized for receiving a signal in such communication system. FIG. 1 depicts such a communication system 100 implementing transmit and receive antenna assemblies. As shown in FIG. 1, a transmitter 101 generates a downlink signal which is amplified through a power amplifier 103. The amplified signal is passed to a remote antenna 105 through an antenna feed 104. The communication system 100 transmits the downlink signal from the antenna 105 to a user of the communication system 100. As also shown in FIG. 1, an uplink signal is received by an antenna 107 and is passed to a receiver 102 through an antenna feed 106. Generally, the antenna feeds 104 and 106 are long coaxial cables.
For downlink diversity, the communication system 100 transmits more than one portion of the downlink signal. The portions of the downlink signal are transmitted at certain time offsets. The actual amount of time offsets, commonly called delay, is selected for an optimum downlink diversity in a multipath environment. The amount of delay is for an amount such that a receiver which receives the downlink signals combines the transmitted signal and the delayed version to improve the reception gain. One example of such a receiver is a RAKE receiver in a Code Division Multiple Access (CDMA) system or an equalized receiver in a Time Division Multiple Access (TDMA) system.
The prior art process of generating a delayed version of a downlink signal transmitted for downlink diversity is complex and not cost effective. For example, in the prior art communication system 100, the transmitter 101 generates the downlink signal and its delayed version. Then, the delayed version is amplified through a power amplifier other than the amplifier 103, and transmitted through a remote antenna other than the antenna 105. Therefore, the prior art requires a plurality of transmitters, a corresponding plurality of power amplifiers and a corresponding plurality of transmitter antennas.
Further difficulty of the prior art for implementing downlink diversity is installing additional transmitters, power amplifiers and antennas in a pre-existing installed system. The communication system 100 very often does not have provision for such an expansion, because the high cost associated with providing the expansion capacity in the communication system 100. Moreover, the expansion capacity is typically not included in the communication system 100 because the need for downlink diversity is not foreseeable at the time of the initial installation.
For an uplink diversity, a receiver, like a RAKE receiver, receives a signal and portions of the signal that are not highly correlated, and combines these signals in the RAKE receiver to improve the receiver gain. A commonly used method in the prior art is that the communication system 100 has a plurality of receiving antennas, like the antenna 107, a plurality of corresponding receivers, like the receiver 102, and a plurality of corresponding antenna feeds for connecting the plurality of receivers to the corresponding antennas.
The receive antennas for uplink diversity should have sufficient spatial diversity in the form of decorrelation which is in the form of amplitude fading and delay spread. However, in certain situations, the delay spread of the signal can be very small and the position of the antennas themselves may not provide enough delay spread. A RAKE receiver has a property that it extracts signal portions that are sufficiently delayed in time, and combines the extracted portions for enhancing the receiver gain. If the delay spread of the signal is too small and the position of the antennas themselves do not provide enough delay spread, the RAKE receiver does not take full advantage of its diversity capability. In a CDMA communication system, a RAKE receiver needs at least a substantial fraction of one chip delay to take fill advantage of its diversity capability.
Therefore, there is a need for a method and apparatus to implement spatial diversity which overcomes the deficiencies of the prior art.